Considering Akira Toriyama made him with distinct Asian Characteristics, and has even stated that his looks would be that of an Asian makes your whole argument completely mute.
Maybe people did not read it, but a few years back, Akira Toriyama would not allow a DBZ movie to be made, because he wanted an Asian Actor to portray Goku, meaning that the race on Earth that would be most distinct to Goku's Saiyan race would be that of an Asian.
Just look at Yamcha, he's supposed to be of asian background as well, and he's actually portrayed right. His looks and features are not far off from Goku manga wise either.
Case and point, we do know that Goku's closest look would be that of an asian. There shouldn't even be any argument here, considering the creator himself has specifically stated this, all of what Goku wears, eats, the way he lives, and even prayed when he was young only cements this notion, along with the creator's own words.
However, the whole notion of "he's a space alien" so we don't know is the biggest load of garbage I've ever heard.
It's as I said before, no different then Superman from outerspace, or Union from outerspace. There all not from Earth, yet we all know that the nationality that resembles them are caucasian.
Much like Goku, and this is the #1 reason why it will fail. However, the notion of this is "Why do they have a Caucasian playing Goku?" The makers of the movie believe that it will "appeal" to the western audience more.
However, that has never helped, having Goku being played by a caucasian is like what they did when they made Genghis Khan back in the 60's, with John Wayne as Genghis Khan. Other characters on the other hand look pretty decent, as to why CYF as Roshi doesn't have his hair shaved makes no sense.